1. Field of the Invention
The present invention relates to a vehicle having an automatic transmission including a plurality of hydraulically operated coupling devices at least one of which is selectively operated to establish a corresponding one of a plurality of speed steps, and in particular to a hydraulic control circuit including a plurality of electromagnetic valve devices which output respective hydraulic pressures to operate the hydraulically operated coupling devices of the automatic transmission. More specifically described, the present invention relates to a fail-safe function of the hydraulic control circuit that enables the vehicle to run even when the electromagnetic valve devices may fall in an all-fail state (so-called “off fail” state) in which none of the electromagnetic valve devices output the respective hydraulic pressures.
2. Discussion of the Related Art
There is known a vehicle having an automatic transmission including a plurality of hydraulically operated coupling devices at least one of which is supplied with at least one hydraulic pressure and is selectively operated to establish a corresponding one of a plurality of speed steps. For example, the vehicle additionally has a plurality of electromagnetic valve devices which output respective hydraulic pressures to be supplied directly to the hydraulically operated coupling devices. In this vehicle, the electromagnetic valve devices are operated, according to respective control signals outputted from, e.g., an electronic control device, so as to output at least one hydraulic pressure to at least one hydraulically operated coupling device and thereby establish a desired speed step of the automatic transmission. Therefore, if the control signals outputted from the electronic control device do not reach the electromagnetic valve devices, for some reason, e.g., breaking of wires or separation of connectors, then the electromagnetic valve devices may not output the respective hydraulic pressures and accordingly may not operate the hydraulically operated coupling devices, so that the automatic transmission may not establish the desired speed step and accordingly the vehicle may not run.
Hence, there has been proposed a technique of operating, when the electromagnetic valve devices fall in the “off-fail” state in which none of the electromagnetic valve devices output the respective hydraulic pressures, at least one hydraulically operated coupling device of the automatic transmission and thereby enabling the vehicle to run. An example of this technique is disclosed by Japanese Patent Publication No. 9-303545. In the technique disclosed by this publication, if the electromagnetic valve devices fall in the “off-fail” state and accordingly the automatic transmission cannot establish a desired speed step, then a line pressure, i.e., a source pressure of a hydraulic control circuit associated with the automatic transmission, that is separate from respective hydraulic passages to supply the respective hydraulic pressures of the electromagnetic valve devices to the hydraulically operated coupling devices, is supplied to at least one hydraulically operated coupling device, so as to establish a pre-selected speed step of the automatic transmission and thereby enable the vehicle to run.
However, the hydraulic control circuit disclosed by the above-indicated publication can establish only the single, pre-selected speed step when the off-fail state of the electromagnetic valve devices occurs. Therefore, depending upon a current state of the vehicle when the off-fail state occurs and the pre-selected speed step for the off-fail state, the vehicle may not be able to run appropriately after the occurrence of the off-fail state. For example, if the off-fail state occurs when the vehicle is running at a high speed, and the pre-selected speed step for the off-fail state is a third speed step as a low or intermediate speed step, then a rotation speed of the engine of the vehicle may be so increased as to exceed an overspeed range; and if the off-fail state occurs when the vehicle is stopped, and the pre-selected speed step for the off-fail state is a fifth speed step as a high speed step, then a driving force may not be so high as to be able to start the vehicle. These problems are more serious to such automatic transmissions which have a greater number of speed steps.